From 1948 until his forced retirement in 1979, the Mexican photographer Enrique Metinides took thousands of images and followed hundreds of stories in and around Mexico City. And what images and stories they were: car wrecks and train derailments, a bi-plane crashed on to a roof, street stabbings and shootings in the park, apartments and petrol stations set alight, earthquakes, accidental explosions, suicides, manslaughters, murder.

Metinides photographed his first corpse when he was 12. A year later, he became an unpaid assistant to the crime photographer of Mexican newspaper La Prensa, and his pictures appeared in La Nota Roja - the red note or, more colloquially, the Bloody News, the best-selling tabloid.

Now almost 70, Metinides is about to hold his first European show. He has said that he based his photographic style on black-and-white action movies, on cops and gangster flicks. Some of his first boyhood photos were of what he saw on the cinema screen, while others were of the car crashes that were always happening outside his father's restaurant. Always known as El Nino - the boy - Metinides got everywhere from the first, hanging around the police station, going to the morgue, not chasing the ambulance but travelling in it as a volunteer with the Red Cross.

Although comparisons with the New York crime-scene photographer Weegee are inevitable, the context, content and style are quite different. In their way, Metinides's photos are like scenes from unmade movies, using a wide-angle lens and daylight flash, the latter in emulation of news photographers he'd seen in the movies. "My first photograph was always the facade of the building where the crime has been committed," he says in an interview in the exhibition's catalogue, "then one of the entrance, the cartridge case, the blood, the overturned drawer, the corpse. That's a film but in still photos."

These images aren't cheap magazine "photoplays". The deaths and disasters are real. Lingering on the blood, the faces of corpses, a murderer's blood-spattered grin, a stabbing victim's pained astonishment, Metinides made himself Mexico's best-known newspaper photographer. Images of such unrelieved and awful intimacy, intensity and apparent salaciousness are difficult for a British audience, but commonplace in Central and South America. They occupy a cultural place we find hard to understand.

Metinides doesn't just show us the mutilated and the dead, the bodies and the blood. He shows us the gathering crowds, the bewildered and transfixed passers-by, the emergency teams as well as the rubber-neckers. In effect, he shows us the city and its people, not just the random and cataclysmic event, but also its effect. He shows us, too, the inexplicable.

Which is not to say in any way that Metinides's photographs are lacking in humanity. Quite the opposite. They are overflowing with humanity. In fact, that is the real trouble with them - they show us too much humanity. In Metinides's images, we don't just see the body dragged out of the water after the drowning, we see the drowned man underwater, the grey corpse hovering at the bottom of the swimming pool. Or a body being dragged to the bank of a river, like some awful bait trawled at the end of a rope, the spectators on the far bank an inverted frieze reflected in the muddy water.

We see things we feel we shouldn't be looking at, but it is hard to drag our eyes away. The dead woman, with her shiny red nails and blonde coiffure, draped over a mangled post after being hit by a car at a pedestrian crossing, her made-up face grim in death, just at the moment when the paramedic is about to cover her with a blanket. The suicide by hanging, dangling from "the tallest tree in Chapultepec Park, unable to bear the fact that her husband has taken their daughter to live with him and his lover". Here, the beautiful tree fills most of the image. The hanged woman is almost a detail, in the soft dappled light at the foot of the tree.

Metinides's images are sometimes made more unsettling by their evident aestheticisation, or perhaps rather the way we place them among other kinds of images, as if to defuse them, render them more acceptable. The man being brought down from the tangle of power lines on the pole - where he fried as he tried to illegally tap into the national grid - looks like an image of Christ's deposition. But aren't paintings and sculptures of Christ on the cross "aestheticised" too? The sequence of shots showing two rescuers attempting to approach and grab a would-be suicide from a stadium gantry (saving him from his wish "to know what death is like") is also a drama of silhouettes and criss-cross girders against the white sky as a human event. It is about the spectacle as much as a lurid, voyeuristic spectacle in itself. In a way, this sequence tells us why we are looking, as much as it is a record or rescue.

The captions are as terse and direct as the images themselves. They give us the context, but also leave us baffled: we are, after all, foreigners here. A woman carries a small box under her arm, as she approaches some men in business suits on the street. We learn that she is a poor woman who has been "forced to leave the morgue in order to buy a coffin for her two-year-old daughter, whose autopsy has been been performed two hours previously".

Other images are deeply enigmatic in another way. In the background of one, we see the derailed train at the mouth of a tunnel. In the foreground, lain on white blankets among the undergrowth is a train worker. Kneeling at his head, amongst the grasses, a uniformed policeman takes notes. It is a surreal image. The blanket is like an opened shroud, and the victim might almost be dreaming. The image is, in fact, like a kind of dream. The cop could almost be drawing, rather than taking a statement. Everything is still, almost like a diorama model, and, inadvertantly, beautifully composed.

So, too, is an incredible photo of a man lying in the street at night, electrocuted by a fallen power line. There he is, flat out in his suit, lit only by the luminous flare of the fizzing wire, which also lights up the curb and silent empty corner. How did Metinides get there, you ask? Why is there no one else on this otherwise dark and empty street? The man, we are told, survived.

Perhaps the image that haunts me most shows a late 1940s sedan rolled on its side in the middle of the road on a bend. If it weren't for the people, you might think it was a kid's toy, knocked over in a game. A man stares at the now vertical underside of the car, as if he'd never seen such a thing before. The car casts a long shadow across the country highway. Two women cross the road in the low angle of late afternoon light. One wears a white dress that picks up the sunlight, as does the sleeve of the man's shirt, the white bodywork and the grinning chrome grille of the car. You imagine the metal ticking as it cools down and the sound of crickets and rustling leaves. The women are walking into their own shadows. The photo has all these white accents: the white dress, the man's shirt, the white car, the whitewashed roadside markers, the white clouds massing over distant mountains. Finally, the white unbroken line painted down the middle of the blacktop, a sweeping cartoon parabola.

It all happened a long time ago, in 1951, somewhere in Puebla State, Mexico. As far as we can see, no one died. The image has the quality of one of those memories one is never quite sure was something one experienced oneself, or was a thing read about and elaborated in the imagination. These photographs seem to be more the beginning of something than a record of something past. This is what makes Metinides such a terrific photographer, even though his subjects are so unrelievedly grim.

Since being ousted from La Prensa, Metinides has not taken a single photograph, though he hasn't exactly retired. He stays in his Mexico City apartment, surrounded by TVs and radios, ceaselessly monitoring the bloody news on the local and satellite channels, videoing second-hand disasters now. His radios are tuned to the police frequencies, and his shelves are stacked with video recordings. He has a collection of thousands of toy ambulances, firetrucks and figures, some arranged in little scenes of rescue and disaster. He also - curiously - keeps a big collection of plastic frogs. Maybe he is trying to explain the world to himself. Which is what we do, too, when we look at these difficult images.

· Enrique Metinides is at The Photgraphers' Gallery, London WC2, from July 24 to September 7. Details: 020-7831 1772.